BACKGROUND:When the Smithy Gang took over Bowser's Keep, they set about reinforcing the Blasters found in the castle so they could withstand more damage. The end result was Big Bertha: they look almost exactly the same as Bill Blasters but they're much more dangerous.
The few Big Berthas that weren't destroyed by Mario and friends were dismantled. Some rumours suggest that Bowser's engineers researched the remaining technology to build better weapons in the future.